Many types of bacteria are equipped for chemotaxis, the movement toward or away from specific chemicals. Investigators of this behavior in enteric bacteria have often emphasized its importance as a model for the sensory physiology or even the central nervous system physiology of higher organisms. The long- term aims of this project are to extend the powerful methods developed in such work toward an understanding of the role that bacterial chemotaxis has in microbial ecology. Previous experiments demonstrate that the chemotaxis of rhizobia to localized sites on the surface of legume roots plays a role in the infection process. This infection initiates a symbiotic association for nitrogen fixation which clearly must be maximized if we are to continue to meet human nutritional needs in a time of decreasing availability of traditional energy sources. Attractant activity of purified chemicals and plant root extracts is assayed in a simple, sort agar plate assay. This assay is made specific by the use of two mutants of Rhizobium meliloti strain SU47 that no longer detect any pure substance tested (primarily sugar and amino acids) but still detect the localized site attractant. The structure of two peptides, isolated from alalfa root tips, that attract these two mutants and the wild type at low concentration will be determined following further improvements in the purification scheme. Mutants that have a specific defect in the response to these plant substances will be isolated and used for tests of the hypothesis that the response has a role in the infection process. A relable technique for permanant marking of small sites will allow further ultrastructural studies of the localized sites of bacterial accumulation. Particular emphasis will be placed on time course studies that can test the hypothesis that these are potential infection sites. The findings of these studies will be relevant for study of bacterial infection in other complex microenvironments including, for example, the mouth and the gut.